The invention relates to a communication device for a field device for transferring output information to a controller, the communication device comprising a passive digital output with a first connection point and a second connection point, a circuit arrangement, which is connected between the first connection point and the second connection point, as well as a control device which is configured to selectively put the circuit arrangement into one of a plurality of switching states according to the output information to be transferred, wherein the communication device is configured to provide, in a state in which the passive digital output is connected to the controller, an electric output signal with a first signal value according to a first communication protocol at the connection points in a first switching state of the circuit arrangement and to provide the electric output signal with a second signal value according to the first communication protocol at the connection points in a second switching state of the circuit arrangement.
The mentioned communication device is used to transfer information from a field device to a controller, for example a superordinate controller, in particular a PLC or SPS. The information transferred from the field device to the controller shall also be denoted as output information. The output information may, for example, be a status confirmation from the field device to the controller.
The field device can typically expend only very little or no energy for the transfer of this output information. This is, for example, due to the fact that the output, at which the output information is provided, is to be used in an explosion-proof region in which the electric power used may not exceed certain threshold values and, for this reason, an actually available energy supply of the field device may not be used or may be used only to a limited extent for this output.
The output for the transfer of the output information is thus configured as a passive output. This means in particular that the energy, by means of which the electric output signal is provided, does not originate from the communication device, but instead from the controller.
For example, the controller has a voltage source, which is connected to the passive output during the transfer of the output information. Depending on the output information to be transferred, the circuit arrangement of the communication device is put into a certain switching state, whereby this causes an electric signal with a certain signal value, for example an electric current with a certain current strength to be set at the passive output. The controller is in turn configured to detect this current strength and to determine the transferred output information therefrom.
The switching state of the circuit arrangement thus serves to set the signal value of the electric output signal. The circuit arrangement has a plurality of switching states, consequently a plurality of signal values can be provided. The signal values are value-discrete such that the output for transferring the output information is a digital output.
The signal values are defined according to a first communication protocol. For example, a plurality of valid current strengths, voltages and/or current-voltage value pairs are defined as signal values by the first communication protocol. The controller, to which the output information should be transferred, is configured according to a communication protocol; i.e., the controller is configured to detect signal values of the electric output signal, the signal values being defined by the communication protocol, and to convert these according to the communication protocol into the output information.
There are different communication protocols, which differ in at least one defined signal value. For example, for communication with a controller configured as a PCL or SPS, a different communication protocol is prescribed than for communication with an NAMUR controller (NAMUR=User Association of Automation Technology in Process Industries).
Compatibility with a plurality of communication protocols is conventionally ensured by a communication device being equipped with a plurality of passive digital outputs, wherein a separate passive digital output is provided for each communication protocol.